Pi-hole has no impact on your data traffic speed. Only the DNS queries go to Pi-hole. The data traffic that follows a DNS query and reply is between the client, the router and the internet with no involvement from Pi-hole.
6 mbps is orders of magnitude faster than is needed for the DNS traffic to/from Pi-hole. DNS queries are a few hundred bytes each.
The USB root hub belongs to your RPi's onboard USB controller.
From the listed devices, I can already tell that your Ethernet adapter is using the (somewhat infamous) DM9601 chipset - also note the 9700 device id.
We are going to use the Bus (001) and Device (002) numerals from your output for the next step:
The following command will show your adapter's supported USB version
(In this forum, you can paste images directly into a post. For textual output, it would be preferred that you paste the text and format it using</> Preformatted text from the menu. )
That's USB 1.1, which translate to a maximum of 12Mbit/s.
That's faster than 10MBit/s standard Ethernet, but a far call from FastEthernet or USB2.
In contrast, USB 2.0 would allow for 480Mbit/s, though most USB2 adapaters are limited to FastEthernet speed (100 Mbit/s).
Many cheap adapters feature that USB ID 0fe6:9700 chipset and claim FastEthernet speed as well as USB2. While those claims are not exactly false (technically, you can plug it into USB2 ports, and as 12Mbit/s exceeds standard Ethernet by 2MBit/s, technically you'd have to negotiate FastEthernet to make use of those), they are grossly misleading - and disappointing for users having expected full FastEthernet speeds.
Early driver support was dodgy as well, but at least that has been fixed in the meantime.
While DNS wouldn't suffer much from those low transfer speeds, downloading updates to your RPi Zero will take the full penalty and take about 10 times as long as over a FastEthernet connection.
If you are just using that adapter for Pi-hole, you may decide to keep it, especially if it was really cheap.
If you paid more than 10 EUR for it and you plan to use it for setting up an RPi as your streaming server, I would try to get a refund and buy a decent adapter.